Internal combustion engines may be charged (e.g., supercharged, turbocharged, etc.), wherein intake air supplied to the cylinders of the engine is at a pressure higher than barometric pressure. Supercharging serves primarily for increasing power. The air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. By means of supercharging in combination with a suitable transmission configuration, it is also possible to realize so-called downspeeding, with which it is likewise possible to achieve a lower specific fuel consumption.
Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
For supercharging, use is in this case made of at least one exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooler may be provided in the intake system downstream of the compressor, by means of which charge-air cooler the compressed charge air is cooled before it enters the at least one cylinder. The cooler lowers the temperature and thereby increases the density of the charge air, such that the cooler also contributes to improved charging of the cylinders, that is to say to a greater air mass. Compression by cooling takes place.
The advantage of the exhaust-gas turbocharger in relation to a mechanical charger is that no mechanical connection for transmitting power exists or is required between charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
On the other hand, in the case of exhaust-gas turbocharging, difficulties are often encountered, inter alia in generating and providing an adequately high charge pressure even at low engine speeds. A torque drop is observed if a particular engine speed is undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. If, for example, the engine speed is reduced, this leads to a smaller exhaust-gas flow and therefore to a lower turbine pressure ratio. As a result, the charge pressure ratio likewise decreases in the direction of lower engine speeds, which equates to a torque drop.
In the prior art, it is sought, using a variety of measures, to improve the torque characteristic of a supercharged internal combustion engine. This is achieved for example by means of a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas flow rate exceeds a critical value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. Said approach has the disadvantage that the supercharging behavior is inadequate at relatively high engine speeds or in the case of relatively large exhaust-gas flow rates. Furthermore, according to the prior art, the blown-off exhaust gas is conducted past the turbine without being used further, and without the energy available in the hot exhaust gas being utilized.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by means of multiple, that is to say at least two, turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, wherein turbines are activated successively with increasing exhaust-gas flow rate.
The torque characteristic may also be advantageously influenced by means of multiple exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows, which considerably improves the torque characteristic in the lower engine speed range. This is achieved by designing the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again upstream of the low-pressure turbine. In the bypass line there is arranged a shut-off element for controlling the exhaust-gas flow conducted past the high-pressure turbine.
With targeted configuration of the supercharging, it is duly also possible for advantages to be achieved in terms of exhaust-gas emissions, for example, in the case of the diesel engine, for nitrogen oxide emissions to be reduced without losses in efficiency, and/or for hydrocarbon emissions to be favorably influenced. To adhere to future limit values for pollutant emissions, however, further measures are necessary.
Here, the focus is on inter alia the reduction of nitrogen oxide emissions, which are of high relevance in particular in diesel engines. Since the formation of nitrogen oxides requires not only an excess of air but rather also high temperatures, one concept for lowering the nitrogen oxide emissions consists in developing combustion processes with lower combustion temperatures.
Here, exhaust-gas recirculation, that is to say the recirculation of exhaust gases from the exhaust-gas discharge system into the intake system, is expedient in achieving this aim, wherein it is possible for the nitrogen oxide emissions to be considerably reduced with increasing exhaust-gas recirculation rate. Here, the exhaust-gas recirculation rate xEGR is determined as xEGR=mEGR/(mEGR+mfresh air), where mEGR denotes the mass of recirculated exhaust gas and mfresh air denotes the supplied fresh air, which has possibly been compressed in a compressor. Exhaust-gas recirculation is also suitable for reducing the emissions of unburned hydrocarbons in the part-load range.
To obtain a considerable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates may be used, which may be of the order of magnitude of xEGR≈60% to 80%.
To be able to realize such high recirculation rates, effective cooling of the exhaust gases for recirculation, with an intense lowering of the exhaust-gas temperature, is indispensable, that is to say such high recirculation rates may not be achievable without lowered exhaust-gas temperatures. A cooler may be provided in the line for exhaust-gas recirculation, which cooler lowers the temperature in the hot exhaust-gas flow and thus increases the density of the exhaust gases. The temperature of the cylinder fresh charge which results upon the mixing of the charge air with the recirculated exhaust gases is likewise reduced in this way, as a result of which the cooler in the recirculation line contributes to improved charging of the cylinders with fresh mixture.
However, the inventors herein have recognized an issue with the above approaches. To be able to cool the large quantities of exhaust gas required for high recirculation rates, and to be able to extract and dissipate the amount of heat that arises here, it is the case that coolers of very large volume may be required, which make dense packaging impossible.
Accordingly, systems and methods are provided herein at least partly address the above issues. In one example, a system comprises an engine including at least one cylinder, an intake system for supplying charge air to the at least one cylinder, an exhaust-gas discharge system for discharging exhaust gas from the at least one cylinder, a first exhaust-gas turbocharger including a first turbine arranged in the exhaust-gas discharge system and a first compressor arranged in the intake system, and an exhaust-gas recirculation (EGR) system. The EGR system includes a line which branches off from the exhaust-gas discharge system and opens into the intake system, a second exhaust-gas turbocharger comprising an EGR turbine arranged in the line on a shaft and an EGR compressor arranged in the line on the shaft upstream of said EGR turbine, and an EGR cooler positioned between the EGR turbine and the EGR compressor.
In this way, the second exhaust-gas turbocharger, arranged in the EGR system and hence also referred to as an EGR turbocharger, may include a compressor upstream in an exhaust gas flow path of a turbine to compress the exhaust gas to a high pressure and then subsequently expand the exhaust gas, thus lowering the temperature of the exhaust gas. An EGR cooler may be disposed intermediate the compressor and turbine, thus further lowering exhaust gas temperatures. Due to the very low temperature of the exhaust gas, high rates of EGR may be provided, thus lowering emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.